Conventional combustion, carried out in the presence of a flame and normally used in hydrocarbon combustion processes such as that of natural gas, is a process which is difficult to control. It takes place in a range of set air/hydrocarbon concentrations and, apart from carbon dioxide and water formation, it produces pollutants such as carbon monoxide and oxides of nitrogen.
The accelerating severity of environmental restrictions on pollutants (oxides of nitrogen, unburned hydrocarbons, carbon monoxide) emitted by combustion processes has meant that new technologies have had to be found which can considerably reduce such emissions. A number of solutions are well known to the skilled person:
Selective reduction of exhaust gases (SCR, Selective Catalytic Reduction). Selective reduction of oxides of nitrogen by ammonia can reduce NO.sub.x concentrations to about 10 ppm. However, this solution requires a run-away use of a special reactor, and the storage and use of ammonia; thus installation costs and operating costs of SCR are high. PA1 Injection of water or steam. Such injection reduces the temperature reached by the combustion gases thus significantly reducing NO.sub.x concentrations to about 50 ppm. The additional cost of such an apparatus is low. However, the operating costs of such an installation are high as the water must be purified prior to injection and because of an overconsumption of fuel due to a reduction in the energy yield. Further, while water injection satisfies current limits, it will not satisfy future limits for NO.sub.x. PA1 A primary lean mixture zone. This technology is based on improving the homogeneity of the air/fuel mixture. It can reduce NO.sub.x emissions to about 50 ppm, but such a reduction is to the detriment of carbon monoxide and unburned hydrocarbon emissions, which increase. PA1 A is at least one element selected from the group constituted by Ba, Ca and Sr, with (0.0.ltoreq.z.ltoreq.0.4); PA1 B is at least one element selected from the group constituted by Mn, Fe, Co, Ni, Cu and Cr, with (x.ltoreq.y.ltoreq.2x); PA1 C is K and/or Rb; and PA1 .alpha.=1-1/2{X-z (X--Y)+xZ -3Y} where X, Y, Z are the respective valencies of elements A, C and B. PA1 A is at least one element selected from the group constituted by Ba, Ca and Sr, with (0.0.ltoreq.z.ltoreq.0.4); PA1 B is at least one element selected from the group constituted by Mn, Fe, Co, Ni, Cu and Cr, with (x.ltoreq.y.ltoreq.2x); PA1 C is at least one element selected from the group constituted by K, Rb and the rare earths; PA1 D is at least one element selected from the group constituted by Au, Ag, Pd, Pt and other precious metals of the platinum group, where x+u.ltoreq.4; and PA1 .alpha.=1-1/2{X-z (X--Y)+xZ+uU-3y-3u} where X, Y, Z and U are the respective valencies of elements A, C and B and D. PA1 European patent application EP-A-0 198 948 which uses: in the 1.sup.st catalytic zone: Pd and Pt and NiO, and in the 2.sup.nd catalytic zone: Pt and Pd; PA1 Japanese patent application JP-A-04/197 443 which uses: in the 1.sup.st catalytic zone: Pd and/or Pt; in the 2.sup.nd catalytic zone: Sr.sub.0.8 La.sub.0.2 MnAl.sub.11 O.sub.19-.alpha. ; and in the 3.sup.rd catalytic zone: Sr.sub.0.8 La.sub.0.2 MnAl.sub.11 O.sub.19-.alpha. ; PA1 International patent applications WO-A-92/9848 and WO-A-92/9849 which use:
Catalytic combustion is a seductive solution by which to respond to the increasing severity of pollutant limits. The catalytic combustion chamber advantageously replaces conventional burners as it allows better control of complete oxidation over a wide range of air/hydrocarbon ratios, thus greatly reducing emissions of oxides of nitrogen, unburned hydrocarbons and carbon monoxide. Further, it can be used to burn a wide variety of compounds.
Thus, as described in particular in the article by D. Reay in "Catalytic Combustion: Current Status and Implications for Energy Efficiency in the Process Industries", Heat Recovery Systems and CHP, 13, No 5, pp 383-390, 1993 and by D. Jones and S. Salfati in Rev. Gen. Therm. Fr. No. 330-331, pp 401-406, June-July 1989, catalytic combustion has a variety of applications: radiant panels and tubes, catalytic afterburners, gas turbines, cogeneration, burners, catalytic sleeves for steam reforming tubes, production of hot gases in the heating field by direct contact and catalytic plate reactors.
In catalytic combustion processes in the fields of energy production and cogeneration, the most widely used reactor configuration is a reactor comprising a plurality of catalytic zones: the catalyst(s) at the inlet are more specifically dedicated to initiating the combustion reaction, while the subsequent catalysts stabilise the combustion reaction at high temperature; the number of catalytic stages (or zones) is adjusted as a function of the conditions imposed by the envisaged application.
Combustion catalysts are generally prepared from a monolithic substrate of ceramic or metal on which a fine support layer is deposited which is constituted by one or more refractory oxides with greater surface area and porosity to that of the monolithic substrate. The active phase, which is essentially composed of metals from the platinum group, is dispersed on the oxide.
As is known to the skilled person, platinum group metals have the highest catalytic activity for hydrocarbon oxidation and thus initiate combustion at a lower temperature than transition metal oxides. Preferably, then, they are used in the first catalytic zones. However, because of the high temperatures reached either during start-up phases or in a steady state, such catalysts degrade, causing a drop in their catalytic performances. Sintering the alumina based support and sintering the active metal phase and/or encapsulating it by the support are the causes which are most frequently cited to explain such degradation.
The drop in the specific surface area of alumina based supports is known to be stabilised effectively by a suitable doping agent. Rare earths and silica are often cited as being among the most effective stabilisers for alumina. Catalysts prepared using this technique have been described in U.S. Pat. No. 4,220,559, among others. In that document, the catalyst includes platinum group metals or transition metals deposited on alumina, an oxide of a metal selected from the group constituted by barium, lanthanum and strontium and an oxide of a metal selected from the group constituted by tin, silicon, zirconium and molybdenum.
In addition, to limit sintering of the active metallic phase, the addition of a variety of stabilisers essentially based on transition metal oxides has been proposed.
Thus U.S. Pat. No. 4,857,499 describes a catalyst comprising a porous support with a pore diameter which is in the range 150 .ANG. to 300 .ANG. and with a proportion by weight with respect to the substrate which is preferably in the range 50 to 200 g/l, an active phase including at least 10% by weight, with respect to the porous support, of a precious metal selected from the group formed by palladium and platinum; a first promoter including at least one element selected from the group constituted by lanthanum, cerium, praseodymium, neodymium, barium, strontium, calcium and oxides thereof, in which the proportion by weight with respect to the porous support is in the range 5% to 20%; a second promoter including at least one element selected from the group formed by magnesium, silicon and oxides thereof, in which the proportion by weight with respect to the active phase is no more than 10%; and a third promoter including at least one element selected from the group constituted by nickel, zirconium, cobalt, iron and manganese, and oxides thereof, in which the proportion by weight with respect to the active phase is no more than 10%. Further, that catalyst can be deposited on a monolithic substrate which is in the group formed by cordierite, mullite, alpha alumina, zirconia, and titanium oxide; the proportion by weight of porous support with respect to the substrate volume is in the range 50 g/l to 200 g/l.
U.S. Pat. No. 4,793,797 describes a catalyst comprising an inorganic support selected from the group constituted by oxides, carbides and nitrides of elements from groups IIa, IIIa and IV of the periodic classification of the elements, or selected from the group constituted by La--.beta.--Al.sub.2 O.sub.3, Nd--.beta.--Al.sub.2 O.sub.3, Ce--.beta.--Al.sub.2 O.sub.3 or Pr--.beta.--Al.sub.2 O.sub.3, at least one precious metal selected from the group constituted by palladium, platinum, rhodium and ruthenium, and at least one oxide of a base metal selected from the group constituted by magnesium, manganese, cobalt, nickel, strontium, niobium, zinc, tin, chromium and zirconium, such that the atomic ratio of base metal to precious metal is in the range 0.1 to 10.
In addition, regarding formulations which can act at high temperature, mixed oxides are generally more resistant than precious metals. Of such oxides, perovskites and more particularly LaMnO.sub.3, LaCoO.sub.3 and La.sub.1-x Sr.sub.x MnO.sub.3 where 0.ltoreq.x.ltoreq.0.2 are important for the catalytic oxidation of hydrocarbons, but their surface area drops rapidly when the temperature goes beyond 800.degree. C. H. Arai et al. have proposed formulations based on hexaaluminates containing manganese which are a good activity/thermal stability compromise, as described in U.S. Pat. No. 4,788,174, in particular. The catalytic combustion catalyst proposed can be represented by the formula: Al.sub.1-z C.sub.z B.sub.x Al.sub.12-y O.sub.19-.alpha., where
H. Arai et al. have also proposed the addition of a precious metal to such catalysts, as described in particular in U.S. Pat. No. 4,959,339. The catalyst proposed is represented by the formula: A.sub.1-z C.sub.z B.sub.x D.sub.u Al.sub.12-y-u O.sub.19-.alpha., (where
Some particularly representative patents concerning combustion reactors with a plurality of catalytic zones are:
in the 1.sup.st catalytic zone: Pd and (Pt or Ag); PA2 in the 2.sup.nd catalytic zone: Pd and (Pt or Ag); and PA2 in the 3.sup.rd catalytic zone: perovskite ABO.sub.3 or an oxide of a metal from group V(Nb or V), group VI (Cr) or group VW (Fe, Co, Ni).
The critical point of a multi-stage process is control of the temperature in the different catalytic stages. If a run-away combustion reaction occurs, the temperature of the catalyst can rapidly reach the adiabatic temperature. It is important to cover the whole of the charge range of a gas turbine. The air-fuel ratio can vary widely from ignition to full charge via the throttle. Using such a catalytic combustion chamber can thus be difficult.
A global approach to the catalytic combustion process, taking into account both the advantages and disadvantages of the catalytic reactor configuration and the catalytic formulation, thus appears to be vital. Further, despite the large amount of development work which has already been carried out, it is still important to find a catalytic reactor configuration--catalytic formulation combination which will satisfy the ever more draconian demands on a combustion process.
The assignee's French patent application FR-A-2 726 774 describes combustion catalysts comprising iron and cerium associated with palladium and/or platinum deposited on a refractory inorganic oxide.
The assignee's French patent FR-B-2 721 837 describes combustion catalysts which essentially have the formula A.sub.1-x B.sub.y C.sub.z Al.sub.12-y-z O.sub.19-.alpha., where A represents at least one element with valency X selected from the group formed by barium, strontium and rare earths; B represents at least one element with valency Y selected from the group formed by Mn, Co and Fe; C represents at least one element selected from the group formed by Mg and Zn; x is 0 to 0.25, y is 0.5 to 3 and z is 0.01 to 3; the sum y+z has a maximum value of 4 and .delta. has a value which, determined as a function of the respective valencies X and Y of elements A and B and the values of x, y and z, is equal to 1-1{(1-x)X+yY-3y-z}.